DEV: code cleanup (#37809)

- Removes double private keyword
- Fixes test mutating json as string, instead of json as hash. Calling
`string["description"] = ""` uses Ruby’s `String#[]=` method which finds
the substring `description` and replaces it with `""`, corrupting the
JSON but leaving it parseable with a `nil` description field instead of
`""`.

lib/onebox/engine/github_issue_onebox.rb

@@ -70,8 +70,6 @@ module Onebox
         }
       end
 
-      private
-
       def load_repo
         load_json("https://api.github.com/repos/#{match[:org]}/#{match[:repo]}")
       end

spec/lib/onebox/engine/github_repo_onebox_spec.rb

@@ -46,9 +46,9 @@ RSpec.describe Onebox::Engine::GithubRepoOnebox do
 
     context "when the repo has no description" do
       let(:response) do
-        resp = onebox_response(described_class.onebox_name)
+        resp = MultiJson.load(onebox_response(described_class.onebox_name))
         resp["description"] = ""
-        resp
+        MultiJson.dump(resp)
       end
 
       it "includes a message about contributing to the repo" do